Xiangwen Wang

Active Pot Control using Alcoa STARprobe

To run an aluminum smelting cell, routine bath sampling and subsequent chemistry analysis are required along with pot temperature measurement. The sampling and analytical process is lengthy, tedious, and very often results are delayed as long as 24 hours. In addition the results are not coupled to other critical information (e.g. noise, automatic resistance adjustments, etc) at the time of the sample. Alcoa STARprobe™, which was previously described (1), corrects these deficiencies while providing a means to more efficiently and effectively control a smelting pot. This paper presents the background philosophy for an advanced control that has been enabled by the new measurement technique. The control method has been applied in multiple plants and demonstration of improved performance will be shown.

Towards On-line Monitoring of Alumina Properties at a Pot Level

Aluminum reduction cells typically use about 1.9 kg of alumina in order to produce 1 kg of aluminum. Hence, for modern reduction cells operating in the 350 to 400 kA range, 5000 to 6000 kg of alumina is fed to reduction cells on a daily basis. However, no information is available in an on-line fashion about the alumina properties fed to the pot. Alumina feeding control systems assume that alumina properties are constant for all pots within a potroom and also over time. Therefore, these control systems aim at controlling alumina concentration dissolved in the bath without accounting for the time varying effects of alumina properties and/or pot condition on alumina dissolution. Based on sampling campaigns, this paper presents evidences of time varying alumina properties impacting its dissolution rate and also proposes a novel approach in order to measure on-line, at the pot, parameters that are related to alumina dissolution.

In‐Situ Formation of Slots in Sederberg Anodes

Traditional carbon anode technology relies on the natural flow of gases from under the anodes during the aluminum reduction process. The anode gas bubbles generated on the bottom surface of carbon anodes during electrolysis are non-conductive and thus increase energy consumption as they increase the electrical resistance in cells. The use of single and multiple bottom anode slots across the entire bottom surface of prebake anodes is now a widely accepted practice to quickly divert anode gases into bottom slots to allow amperage creep. The slots have the potential to save about three percent of the energy required in the process depending on the number and design of slots. It has now been demonstrated in this work that vertical non-continuous slots can also be formed in-situ in self-baking VS Soderberg anodes by vertically inserting four rows/or layers of multiple aluminum plates into the top surface of the anode during charging anode carbon paste to cells. Extended plant tests confirm that these multiple slots significantly reduce the electrical resistance, lower the cell voltage, and thereby reduce the cell energy consumption in VS Soderberg cells; for example, the pot noise was found to be reduced 40%, (0.04–0.05 V) compared with traditional low noise Soderberg cells; pot noise was reduced by 80% (-0.200 V) when compared to high noise Soderberg cells.

Improvement of Alumina Dissolution Rate through Alumina Feeder Pipe Modification

Aluminum reduction cells use about 1.9 kg of alumina in order to produce 1 kg of aluminum. That is, for modern reduction cells operating in the 350 to 400 kA range, 5000 to 6000 kg of alumina is fed daily. Considering that 5000 to 10000 kg of molten bath is available to dissolve the alumina, the dissolution rate is an important factor in order to avoid muck and enable alumina feed control system to operate within the 2 to 5% alumina concentration. However, on top of cell status, alumina properties have an impact on alumina dissolution rate. Hence, supplier changes and/or segregation of alumina within the delivery system may have negative impact on alumina dissolution rate leading to muck and/or anode effects. This paper discusses modification to an alumina feeder pipe promoting the dissolution rate. Promising results obtained during trial in a pilot plant section are presented and discussed.